A surface-treated metal plate which is used for containers, automobiles, home electrical appliances, building materials, and various other applications is conventionally often treated to prevent rust by chromate using hexavalent chromate etc. Furthermore, the practice has also been to apply a 1 μm or so thin film coating made of an organic resin so as to impart a high degree of corrosion resistance, fingermark resistance, scratch resistance, lubricity, etc. for use or to further apply a coating over this for use.
However, in recent years, from the viewpoint of environmental protection, tough restrictions have been applied on the use of hexavalent chromate and other environmental load substances. For this reason, there has been active research on alternative technologies to chromate treatment. For example, PLT 1 discloses art relating to a surface-treated steel plate with lubricity where a urethane resin or other aqueous resin, organic lubricant, silane coupling agent, etc. are blended in the coating, while PLT 2 discloses art relating to chromate-free treated metal sheet or plated metal sheet made of mainly zinc and aluminum which is coated with a water-based resin containing a thiocarbonyl group-containing compound and phosphoric acid ions and, furthermore, an aqueous dispersion silica.
In such chromate treatment alternative technologies, in recent years, for the purpose of use for various applications and under tougher conditions, realization of corrosion resistance, design, workability, worked corrosion resistance, solvent resistance, alkali resistance, conductivity, wet adhesion, scratch resistance, and other numerous functions simultaneously at a higher level has been sought. To deal with such a demand, for example, PLT 3 discloses the art of applying a cross-linked resin matrix coating which decreases the carboxyl acid groups and instead introduces hydroxyl groups so as to simultaneously impart alkali resistance, corrosion resistance, solvent resistance, and scratch resistance.
As art for simultaneously imparting several different functions in this way to a high level, a coated metal plate having a multilayer coating structure which divides the necessary functions among a plurality of coating layers has been proposed. As art which divides functions among a large number of layers in this way, for example, PLT 4 which provides a coating comprised of a top layer of an amine-modified phenol resin and a bottom layer of a silica-containing resin and which imparts corrosion resistance and top coating adhesion may be mentioned. In the case of the multilayer coating described in PLT 4, the interlayer adhesion of one coating layer and another coating layer is maintained by the chemical bonds between the coatings and the hydrogen bonds or the Van de Waals force and other physical bonds. However, in a multilayer structure aimed at division of functions, since coatings of different performances are laminated, the resin ingredients and additives differ and differences occur in the polarities and surface energies of the coatings. In this case, the physical bonding force between the coatings easily becomes extremely weak. Securing the interlayer adhesion of one coating layer and another coating layer becomes an issue. If the interlayer adhesion is insufficient, the workability, corrosion resistance, and other aspects of performance also can no longer be sufficiently exhibited. Further, blistering accompanying the corrosion and other defects in appearance become pronounced.
As art for improving the adhesion between coating layers of multilayer coatings, for example, as described in PLT 5, the art of introducing melamine to the bottom layer has been disclosed. However, it is generally known that melamine has a detrimental effect on performances other than adhesion, for example, workability.
Further, for example, PLT 6 discloses the art of using part of the thermoplastic resin particles which are dispersed in the bottom layer coating to form projections which penetrate to the inside of the top layer coating in wedge shapes so as to improve the interlayer adhesion between the bottom layer and the top layer. However, with this art, only the adhesion near the wedges formed by the thermoplastic resin particles is improved, so it is difficult to sufficiently obtain interlayer adhesion.
Further, for example, as described in PLT 7, the art is disclosed of simultaneously coating the top layer and the bottom layer by the multilayer coating method or the wet-on-wet method so as to use the anchor effect of the fine surface relief at the interface to improve the interlayer adhesion. However, the effect of improvement of the interlayer adhesion by the surface relief of the extent described in PLT 7 is slight. The necessary interlayer adhesion cannot be obtained.
PLT 8 discloses the art of applying a top layer of a specific composition in the state where a bottom layer has not completely cured (semicured state) and dissolving part of the bottom layer at the top layer so as to improve the interlayer adhesion. However, this art has the problem that it is also possible to utilize only a specific coating and lacks general applicability.